Polyvinyl chloride has enjoyed widespread use for molding various products because of its low cost and ability to be compounded with a variety of additives to obtain plastics having a wide range of physical properties and thus to satisfy a wide variety of needs. Most polyvinyl chloride is processed by calendaring, by extruding or by compression molding. The injection molding process, which gives better product uniformity, and higher automation capability, became practical for polyvinyl chloride with the advent of screw injection molding machines. Injection molding operates by forcing a molten plastic composition under high shear through a small orifice into a cold mold and then allowing the plastic to cool to a solid. The pressure used during injection molding may be about 20,000 pounds per square inch (1406 kilograms per centimeter) under mold pressures of 1-5 tons/in.sup.2 (1.57-7.87 kilograms per square mm). In a screw injection molding machine, pellets of the plastic composition are fed into the feed end of the screw where the plastic is melted and thoroughly mixed and forced through a nozzle at the discharge end which injects the molten material into the mold. When the plastic has solidified, the mold is opened and the molded part removed.
Polyvinyl chloride injection molding compositions are known which are more than adequate for forming most articles; however, known molding compositions are inadequate when large articles having very fine details are to be made. For example, Clemens in U.S. Pat. No. 3,842,194 has described a 12 inch (30.5 cm) diameter information disc having a spiral groove with a pitch of from 5,000-10,000 grooves per inch (1,968-3,937 grooves per centimeter) which has video information in the form of a surface relief pattern in the groove. The pattern elements are very small, on the order of sub-micron dimensions. Color and audio information are also part of the complex information pattern. Such discs have conventionally been made by compression molding, which requires separate steps of fabricating the molding composition and pelletizing it, remelting to form a preform of the desired size and finally molding the material under pressure in a hot mold and cooling to solidify the plastic. Attempts to prepare a molding composition from which a video disc can be injection molded have failed heretofore because fidelity of the information pattern could not be maintained along the whole surface of the disc. In particular, the molten plastic, which was injected into the center of a cool mold, tended to harden before it spread to the outer edges of the disc. To eliminate this problem, the molding composition must be heated to comparatively high temperatures, where both the molding composition itself and the additives tend to be unstable, forming volatile byproducts which adversely affect the surface quality of the molded article. At high temperatures the ingredients also tend to separate, evidenced by staining. The high shear rates encountered during injection molding also cause separation of the components such as stabilizers and lubricants, also causing surface staining and inhomogeneities. Thus an improved molding composition, which can be employed to injection mold large articles with very fine surface details, has been sought.